I can't remember the exact numbers but dextrose is almost 100% fermentable and DME is quite a bit less which is why you need more to achieve the same results. Someone can probably post the actual numbers, I can't remember off the top of my head.

Corn sugar is also noticeably more dense than DME and so a given mass of corn sugar will take up less space than that same mass of corn sugar. Think about a pile of large rocks sitting next to a pile of pebbles, which has more air in the pile? Now, shrink those down, the DME is the rocks and the corn sugar is the pebbles.